The feed screw in feeders which dispense flowable substances (e.g. powders, pellets, granules, fibers and flakes) is driven to rotate by a motor and suitable gearing and the like. As the flowable substance is discharged to the feed screw from a storage hopper positioned above the feed screw, the helical arrangement of the feed screw advances the flowable substance to a dispensing opening at the end of a trough in which the feed screw is positioned.
The torque imparted to the feed screw must be adequate to advance the load of flowable substance dropped into the trough to the dispensing opening. Also, this torque should be sufficient to overcome a jammed condition of the flowable substance.
In addition, the feeder should be arranged for easy insertion and removal of the feed screw, so that different feed screws can be used for handling different flowable substances. The feed screws also are removed for cleaning either as a routine matter or when switching use of the feeder from one flowable substance to another.
A further requirement, tied to the feature of removability of the feed screw, is that the feed screw be retained against unintended axial movement of the feed screw caused, for example, by its own weight when the feeder is set with the feed screw disposed at an angle below horizontal. It is necessary to optimize between adequate retention against unintended axial movement of the feed screw and the facility of relatively easy axial movement for insertion and removal of the feed screw. Also, the feeder, preferably, is arranged, so that the individual who inserts the feed screw receives a signal that the feed screw has, in fact, been inserted properly.
A common use of feeders is to handle certain flowable substances under extremely high sanitary conditions. Under such circumstances, the feeder parts contacted by the flowable substance or those feeder parts to which the flowable substance can migrate should be free of blind cracks or crevices in which the flowable substance can collect.
Among the arrangements used previously to couple a feed screw to a drive system is a threaded connection. The driven end of the feed screw has a thread running in a direction opposite from the helix of the feed screw. The threaded connection makes possible the insertion and removal of the feed screw. There ares however, certain shortcomings of such threaded connections. Often, it is very difficult to remove the feed screw because the feed screw threaded end is jammed against its mating thread during operation of the feeder, so that the break loose torque is high. In certain feeders having the feed screws connected to the drive system by a threaded connection, the drive motor or some other part in the drive system must be provided with a brake which opposes the tendency for rotation as the feed screw is turned for removal. Another shortcoming of such threaded connections is the difficulty of insertion and removal of multiple feed screws. Typically, the feed screws are positioned in such close proximity to another that the helix of one fits within the helix of another. Thus, great care must be taken and much time is required to insert or remove the feed screws without movement of one obstructing movement of the other.
Bayonet arrangements also have been used previously to couple a feed screw to a drive system. For insertion, the feed screw first is moved axially and then it is turned 90.degree. to lock the feed screw in place. For removals the feed screw first is turned 900 in the opposite direction to release the feed screw and when it is moved axially. Bayonet arrangements also suffer from certain shortcomings. One is that they are expensive to fabricate because they require precise machining operations to produce mating surfaces which can accommodate the axial and rotational relative movements. Another shortcoming of bayonet arrangements is weakening of the parts because of the required configurations resulting in either reduced load carrying capability or fabricating larger parts which can handle the expected loads. A third shortcoming of bayonet arrangements is the distinct possibility of collection of the flowable substance in blind cracks or crevices. With bayonet arrangements involving a turning movement as part of the insertion or removal operation, there can be a certain amount of difficulty in the insertion or removal of multiple feed screws but to a lesser extent than with threaded connections because the bayonet arrangement requires only a single 90.degree. movement, whereas the threaded connection requires multiple 360.degree. turns.
Male and female mating parts having complementary polygonal mating surfaces also have been used previously to couple a feed screw to a drive system. Such arrangements also suffer from certain shortcomings. First, because of the difficulty in forming a recess for the particular cross-sectional shapes selected, the depth of the recess is limited, resulting in limited axial support and stability for the feed screw. Second, in order to provide axial retention against unintended axial movement of the feed screw, a long screw, extending through a long bore in either the feed screw or the drive shaft for the feed screw was provided for engagement with the other part. Third, with non-circular mating part, alignment for insertion is difficult.